This invention concerns the purification of 1,12-dodecanedioic acid that contains iron and other color-forming impurities.
U.S. Pat. No. 2,601,233 discloses that 1,12-dodecanedioic acid can be produced by a reductive coupling treatment of an organic peroxide, such as the addition product obtained by the reaction of cyclohexanone and hydrogen peroxide, with an appropriate reducing agent, such as ferrous iron compound. Such a treatment is referred to herein as an "iron-reductive-coupling step." According to U.S. Pat. No. 3,907,883, an iron-reductive-coupling step can be carried out by a process in which a solution of the organic peroxide in a suitable organic solvent is thoroughly mixed with an aqueous solution of a chelated ferrous iron compound, to form the desired dodecanedioic acid by reductive coupling of the organic peroxide and oxidation of the chelated ferrous iron to the ferric state. Two phases are formed in the mixture; an organic solvent phase which contains the desired 1,12-dodecanedioic acid product and an aqueous phase which contains the oxidized or spent reductant solution. The two phases can be separated quite readily, for example by decantation. It is suggested that the dodecanedioic acid product can then be recovered by crystallization from the organic layer or the organic solvent can be steam-distilled and the desired product crystallized from the aqueous tails.
The dodecanedioic acid recovered from the above-described processes is useful, for example, for conversion into polyamides or in the manufacture of plastics. However, it has been found that undesirable quantities of iron and color-forming organic impurities are often present in the acid. Such impurities are detrimental, especially when the dodecanedioic acid is to be converted to polyamide polymers intended for the manufacture of high-quality textile fibers and filaments.